Not applicable.
Not applicable.
The present invention is generally related to optical amplifiers, and in particular provides an amplifier integrated with filters for use in wavelength division multiplexed optical communication networks.
Optical fiber networks are used in a variety of applications, such as optical tele-communication and data transmission systems. Optical fiber networks use optical fibers as transmission lines for carrying light signals. The light signals propagate down the fiber from one location to another, analogous to electrical signals traveling down a wire or cable from one location to another. Optical fibers are used in a variety of applications, such as local loops and xe2x80x9clong haulxe2x80x9d transmission lines. Long haul transmission lines might carry signals between cities or across oceans, for example.
Optical fibers can carry a single channel, or many channels can be multiplexed onto a single fiber. Multiplexing is desirable because it allows more information to be carried on a single fiber. In wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d), a number of channels are carried on a single optical fiber. A channel is typically defined as a frequency (wavelength) of light that can be modulated to carry information. Networks are typically designed to allocate a portion of the spectrum about a center frequency for signal transmission. For example, a channel might be allocated xc2x112.5 GHz on either side of the channel center frequency in a particular system, thus providing the channel with a xe2x80x9cwidthxe2x80x9d of 25 GHz., even though the channels are spaced 100 GHz apart. Other systems may require or allow a narrower or wider channel widths or spacings.
With increased use of the Internet and other communication technologies, the demand for increased transmission capacity continues to grow. Dense wavelength division multiplexing (xe2x80x9cDWDMxe2x80x9d) technology has emerged as an efficient and economical approach to utilize optical fiber capacity For example, a single-mode fiber has about 300 nm of available bandwidth. In many systems, only a portion of that bandwidth is occupied (used).
Different telecommunication companies utilize the bandwidth available on an optic fiber transmission line in different fashions, for example by combining time division multiplexing (xe2x80x9cTDMxe2x80x9d) with DWDM techniques. Some optical telecommunication equipment providers are planning on DWDM systems with up to 160 channels, while others are planning up to 80 channels, within various nominal frequency bands, such as the xe2x80x9c10 GHz bandxe2x80x9d or the xe2x80x9c40 GHz bandxe2x80x9d.
A challenging task of an optical networking system is infrastructure planning. Typically, system engineers need to know the maximum capacity (number of channels) of the transmission system and be able to implement the system with available technology. As the system demand grows, it is generally desirable to be able to expand capacity without distrupting existing traffic flow, much like the challenge of widening a busy highway. A particular challenge arises from the need to periodically amplify optical signals to compensate for signal losses as the signal propagates down an optical fiber.
In many optical transmission systems, provisions are made to amplify the signal (channels) at regular intervals. For example, amplification might be provided every 100 km along a long haul optical fiber path to account for signal loss (attenuation) as the optical signal propagates along the optical fiber. Many approaches have been developed to amplify the optical signals in an optical transmission system.
One early approach periodically converted the optical signal to an electronic signal, amplified the electronic signal, and re-generated the optical signal. This approach was cumbersome and required a relatively large number of components, making it prone to failure. It also typically required compromises in the amount of information (bandwidth) that could be amplified through a given path because the upper frequency limit of an electronic amplifier is generally much less than the available frequency bandwidth carried on an optical fiber.
Further approaches utilized light amplifiers to directly amplify the optical signal. There are many types of light amplifiers adaptable for use with optical communication networks; however, a common type is the doped fiber light amplifier. In a doped fiber light amplifier, an element(s), such as erbium (Eb) is added to the composition of the glass that the fiber is made from. The dopant generally provides energy levels, or states, that can be occupied by photons in the glass fiber. The doped fiber is xe2x80x9cpumpedxe2x80x9d with an external light source, such as a pump laser diode, and this pump light is used to amplify the optical signal.
Erbium-doped fiber amplifiers (xe2x80x9cEDFA""sxe2x80x9d) provide a gain bandwidth suitable for simultaneously amplifying a number of optical channels, and one approach to amplifying the light signal is to amplify all the channels at once in a single broad-band EDFA. This is desirable because it requires only a single amplifier and associated pump circuitry, but has disadvantages, too. First, the gain provided by an EDFA is not uniform for all frequencies (channels). This can result in unequal amplification of the channels, known as gain tilt. In a transmission system where amplification is repeated several times between the source and eventual receiver, the difference in signal level between channels can build to unacceptable levels, since the gain difference generally repeats at each amplifier stage. Second, the gain provided to any one channel is typically somewhat dependant on the total signal the EDFA is amplifying. In other words, the total power needed by the amplifier increases with increasing channel count. For example, at a given pump power, if an EDFA initially amplifies four channels, and the signal traffic on the transmission fiber is expanded to eight channels, the gain of the four original channels will generally be higher than the gain of those channels when the amplifier carries the additional signal traffic. While the light amplifier can sometimes be pumped for higher gain to accommodate the increased number of channels, higher pumping often injects more noise, such as amplified spontaneous emissions (xe2x80x9cASExe2x80x9d), as well as requiring more power to the pump source, often accelerating failure.
One approach to avoid some of the problems associated with broadband amplification of the entire transmission spectrum has been to de-multiplex the transmission spectrum into a number of segments, and to route each segment through an amplifier path. After the segment of the transmission spectrum has been amplified, the optical signals are sent to a multiplexer and coupled onto the output transmission fiber. Such systems allow the use of amplifiers optimized for performance over a portion of the band, each amplifier operating more or less independently from the others. However, providing several amplifiers to cover the transmission spectrum increases the component count, and typically increases the volume required for the amplifiers and associated circuitry.
Accordingly, it is desirable to provide a compact optical amplifier for use in WDM optical transmission systems and other applications. It is further desirable that any such amplifier make efficient use of the pumping energy to reduce the power requirement and necessary volume of associated pump circuitry, and to do so with low added noise.
The present invention provides an integrated modular optical amplifier for use in optical communication systems and similar applications. Wavelength selective filters are integrated with an amplifier to provide a component for use in optical amplifier arrays. In a particular embodiment, a wavelength selective input filter transmits a selected portion of an input light signal to an amplifier stage of the integrated modular amplifier and reflects a remaining portion of the input light signal to an express fiber. The selected portion is amplified and optically coupled to a wavelength selective output filter that transmits the amplified signal to an output fiber and reflects a common optical signal arriving on a common fiber to the output fiber. In a further embodiment, amplification is provided in two stages, with a center tap between the stages allowing access to the selected light signal for signal modification, such as level adjustment or dispersion compensation, before amplification by the second stage. In yet a further embodiment, a single pump light source is coupled to both amplification stages, with a pump interconnect fiber, for example.
In a specific embodiment, the integrated modular amplifier is provided as an essentially cylindrical component that is compact and robust. Use of ferrules containing fiber ends, in conjunction with collimators and other optical elements, reduces the number of required fusion splices required to implement the integrated modular amplifier.